Why Relays Are Used in Power Systems? Working, Types and Importance

Search Description: Learn why relays are used in power systems, how protective relays work with circuit breakers, types of relays, applications, and importance in simple language.

Introduction

A power system is a large electrical network that includes generation, transmission, distribution, transformers, substations, feeders, motors, and other expensive equipment. Since this network carries high voltage and high current, any fault can damage equipment and may also create danger for human life.

This is why protection is very important in power systems. One of the most important protection devices used in electrical networks is the protective relay. A relay detects abnormal conditions and gives a signal to the circuit breaker to disconnect the faulty part from the healthy system.

What is a Relay in a Power System?

A relay is a protective device that continuously monitors electrical quantities such as current, voltage, frequency, power, and phase angle. When any abnormal condition occurs, the relay senses it and operates the trip circuit of the circuit breaker.

In simple words, the relay acts like the brain of the protection system, while the circuit breaker acts like the muscle. The relay detects the fault, and the circuit breaker physically opens the circuit.

Why Are Relays Needed in Power Systems?

Relays are used in power systems because faults and abnormal conditions cannot be ignored. A large fault current can burn conductors, damage transformers, trip generators, create fire, and disturb the whole electrical network.

• To detect faults quickly.
• To protect costly equipment like transformers, generators, motors, and transmission lines.
• To isolate only the faulty section from the healthy section.
• To improve safety for people and equipment.
• To maintain continuity of power supply.
• To reduce damage caused by short-circuit current.
• To provide alarm during abnormal operating conditions.

How Relay and Circuit Breaker Work Together

A relay alone cannot interrupt a heavy fault current. It only senses the fault and sends a command. The circuit breaker performs the actual switching operation.

The working process is simple:

1. A fault occurs in the power system.
2. The current, voltage, or frequency becomes abnormal.
3. The relay detects this abnormal condition.
4. The relay closes the trip circuit.
5. The trip coil of the circuit breaker gets energized.
6. The circuit breaker opens and isolates the faulty part.
7. The healthy part of the system continues to operate safely.

Example: Overcurrent Fault Protection

Suppose a short circuit occurs on a transmission line. Due to this fault, a very large current starts flowing through the line. If this current is not interrupted quickly, it can damage conductors, insulators, transformers, and switchgear.

In this case, an overcurrent relay senses the excessive current and sends a trip signal to the circuit breaker. The circuit breaker opens and disconnects the faulty line from the system.

Important Types of Relays Used in Power Systems

1. Overcurrent Relay

This relay operates when current exceeds a preset value. It is commonly used for feeder protection, transformer protection, and motor protection.

2. Earth Fault Relay

This relay detects leakage current or ground faults. It protects equipment and people from earth-fault conditions.

3. Overvoltage Relay

It operates when voltage rises above a safe limit. Overvoltage can damage insulation and electrical equipment.

4. Undervoltage Relay

This relay operates when voltage falls below a specified value. Low voltage can affect motors, industrial loads, and sensitive equipment.

5. Overfrequency and Underfrequency Relay

These relays are used when the system frequency goes above or below the safe operating range. They are important in power generation and grid stability.

6. Differential Relay

Differential relays compare currents entering and leaving a protected zone. They are widely used for transformer, generator, busbar, and motor protection.

7. Buchholz Relay

Buchholz relay is a mechanical relay used in oil-immersed transformers. It detects internal faults by sensing gas formation or sudden oil movement inside the transformer tank.

8. Pressure and Density Relays

These relays are used in equipment like SF6 circuit breakers and pneumatic circuit breakers. They monitor gas pressure, air pressure, or density conditions.

Trip Relay and Non-Trip Relay

Trip Relay

A trip relay sends a command to open the circuit breaker. It is used when immediate disconnection of the faulty part is required.

Non-Trip Relay

A non-trip relay does not open the circuit breaker. It only gives an alarm or warning. Such relays are useful when the abnormal condition is not immediately dangerous but still needs attention.

Why Relays Improve Power System Reliability

Relays help maintain system reliability because they isolate only the faulty section and allow the healthy portion to remain in service. Without proper relays, a small fault can spread and cause a large blackout.

Modern digital and numerical relays are even more advanced. They can measure many parameters, record fault data, communicate with control centers, and support smart grid protection.

Modern Applications of Protective Relays

• Transmission line protection
• Transformer protection
• Generator protection
• Motor protection
• Busbar protection
• Substation automation
• Smart grid monitoring
• Renewable energy protection
• Industrial power system protection

Beginner Notes

• A relay detects the fault.
• A circuit breaker interrupts the fault current.
• Relays protect equipment and improve safety.
• Relays can work based on current, voltage, frequency, pressure, gas, or oil movement.
• Modern relays are digital and can communicate with control systems.

Frequently Asked Questions

Why are relays used in power systems?

Relays are used to detect faults and abnormal conditions in power systems. They send a trip signal to the circuit breaker to isolate the faulty section.

Can a relay break fault current directly?

No. A relay only detects the fault and gives a command. The circuit breaker opens the circuit and interrupts the fault current.

What is the difference between relay and circuit breaker?

A relay is a sensing and decision-making device, while a circuit breaker is a switching device that opens the faulty circuit.

What is a non-trip relay?

A non-trip relay gives only an alarm or warning. It does not trip the circuit breaker.

Which relay is used in transformers?

Transformers commonly use Buchholz relay, differential relay, overcurrent relay, earth fault relay, and temperature protection devices.

Conclusion

Relays are essential for safe and reliable power system operation. They detect faults, abnormal voltage, abnormal frequency, pressure changes, oil movement, and other unsafe conditions. By working with circuit breakers, relays help protect costly equipment and prevent faults from spreading across the network.

For beginners, the most important point to remember is simple: the relay detects the problem, and the circuit breaker removes the problem from the system.